1. Field of the Invention
The invention relates to a method for selectively separating mixed synthetic organic materials such as filled or unfilled polymers and/or copolymers in the form of wastes and particularly recyclable domestic wastes for reuse, these synthetic organic materials originating from the destruction of cars and obsolete durable consumer products at the end of their lives by grinding, the said selective separation method consisting of separating these materials at a chosen density threshold in a dense medium formed from non-viscous separative liquid suspensions which consist of powder particles dispersed in an aqueous phase and stabilised at a density threshold value chosen so as to provoke the selective separation of a determined fraction of the mixture of waste materials to be separated.
2. Description of Related Art
The waste synthetic organic materials concerned generally originate from the destruction of cars and obsolete durable consumer products at the end of their lives by grinding, in which a large number of different types of synthetic organic materials are present, including filled or unfilled polymers and/or copolymers, with or without fire retardant, with or without additives, are considered to be reusable, and in which a large number of other materials are considered as being unwanted contaminants such as metals, minerals and other various contaminants that must firstly be eliminated. Other wastes, such as mixed industrial wastes containing synthetic organic materials and packaging wastes such as polymers and/or copolymers originating from municipal tips and also containing mixed polymer materials can also be considered as being potentially reusable.
In industries for recycling waste synthetic organic materials to be reused, various methods are used for separating more or less polluting and more or less polluted stream constituents with a variable concentration of reusable synthetic organic materials that have to be separated from the pollutants, concentrated and sorted by homogeneous streams of the different types of synthetic organic materials present, for example such as polyethylene, polypropylene, polystyrene (PS), acrylonitrile-butadiene-styrene (ABS) copolymer, polyamides (PA), polyvinyl chloride (PVC), polyesters, polyurethane, polycarbonate, acrylic or metacrylic or other copolymers, all polymers that may or may not be filled, used with additives, and with or without fire retardants.
These known methods can currently be used to extract and separate streams to be treated, composed of mixes of synthetic organic materials to be reused and contaminating materials to be eliminated, these streams including:                a phase of synthetic organic materials with a density less than 1,        a phase of synthetic organic materials with a density greater than or equal to 1,        a phase of synthetic organic materials, for example consisting of polyethylene, polyurethane foams, film waste, wire waste and others,        a phase of contaminating materials to be eliminated, consisting of constituents that cannot be reused in such separation and reuse methods, for example sand, glass debris, wood debris, remaining metals and others.        
One of these methods for separation of all types of polymer materials originating from the destruction of cars and/or other objects at the end of their lives by grinding (described in European patent EP 0918606 B) consists of performing a grinding step to fragment synthetic organic materials, then preferably carrying out a mechanical separation based on a shape factor, followed by a first density separation step in which all synthetic organic materials to be reused are separated into two streams:                the first with a density less than 1, particularly and for example including unfilled polyolefins such as polyethylenes (d=0.92 to 0.95), polypropylenes (d about 0.9), ethylene and vinylacetate copolymers, ethylene-propylene rubber (E.P.R) copolymers, ethylene-propylene-diene-monomer (E.P.D.M.) copolymers, polyethylene (PE) foams, polypropylene (PP) foams, polyurethane (PU) foams and others,        the other with a density greater than or equal to 1, and for example including particularly:                    polystyrene: unfilled PS (d about 1.05)            acrylonitrile-butadiene-styrene copolymers: unfilled ABS (d about 1.07)            polyvinyl chlorides: unfilled rigid PVC (d about 1.30 to 1.40) and filled rigid PVC (d about 1.40 to 1.55), plastified PVC such as plastisol and PVC foams            polycarbonates: unfilled PC with density d=1.20 to 1.24, filled PC reinforced with 20% glass fibre with density d=1.3, or filled PC reinforced with 30% of glass fibres with density d=1.44            thermoplastic rubbers, except for thermosetting honeycombed rubbers            polyurethanes: filled PU (d=about 1.21)            Talc-filled polypropylenes (PP/talc filled with between 5% and 40% of talc)            filled polyethylenes: (Filled PE containing between 2% and 40% of fillers)            unsaturated polyesters (d varying from about 1.10 to 1.13)            saturated filled or unfilled polyesters (d≧1.20) (frequently filled with glass fibres)            polyamides: filled or unfilled PA6 (d=1.13), PA6.6 (d=1.14), PA6.10 (d=1.08), PA11 (d=1.04), PA12 (d=1.02),            polymethyl methacrylates: PMMA (d=1.18)            others.                        
These two streams are treated later to extract each component from them, to separate them into homogenous streams and treat them to make them into formulated pellets that can be used directly for plastic transformation processes.
If the stream of the mix of reusable synthetic organic materials with a density of less than 1 can be efficiently treated by a series of separation steps in aqueous medium using baths with an appropriate density to enable a fine selection of the different polymer compounds present in the said stream, this is not the case for the stream of the mixture of organic materials with a density greater than or equal to 1 for which it is difficult to separate the different synthetic organic materials.
For this second stream that is composed of a mix of filled or unfilled polymers and/or copolymers, with densities varying from 1 to about 1.6, the various synthetic organic materials present in the mix are density separated in hydraulic separators in which the liquid separation medium consists of water, surfactants and mineral compounds such as clays and particularly bentonite and possibly compounds soluble in water such as mineral salts, compounds used to increase the density of the liquid phase and increase it until its density enables separation in principle of the various organic synthetic materials into two distinct phases, one supernatant and the other sinking in the bottom of the separator, each phase forming a new stream for which a new density separation is carried out in turn.
However it is found that the liquid separation media are not sufficiently stable in practice to enable accurate selective separations by density, in other words providing very homogeneous separations into types of sorted materials, particularly in density separation installations in use in industries for processing waste synthetic organic materials to be reused, since streams output from the separation can be mixes of several materials for which the corresponding densities are very similar to each other.
Several particularly annoying phenomena are observed that occur in these liquid density separation media; these annoying phenomena are genuine disadvantages, and are as follows:                a variation of the rheology of the said liquid media, that is demonstrated by a disturbing variation of their viscosity towards a more fluid or a more pasty state,        a drift or a variation of the apparent density of liquid media, since the density cannot remain stable at the initially chosen threshold for good separation of waste synthetic organic materials to be reused, this drift (variation) changing the composition of the streams separated by a dense liquid medium,        settlement in time of a fraction of the mineral compounds dispersed in the aqueous medium to create the chosen density, partly causing the change in the density of the liquid separation medium,        quasi-impossibility of accurately separating filled or unfilled waste synthetic organic materials to be reused, as soon as the density separation of the different materials has to be done for a density difference Δ=|0.01|, in other words within an interval around the chosen density “ds” equal to ds+/−0.005.        
Thus, there is an undoubted and important problem about very precise density separation of synthetic organic waste materials to be reused such as filled or unfilled polymers and/or copolymers, consisting of used wastes to be recycled originating from the destruction of cars and obsolete durable consumer products at the end of their lives by grinding, to obtain separated homogeneous material streams without a drift in the density and mix of the selected materials.
A large amount of research work has been done on the use a dense liquid medium to achieve the best possible separation of solid materials with a density equal to at least 1 or very much more than 1 and that are difficult from each other, and there are many industrial applications.
The dense liquid medium used in the state of the art with a density usually greater than 1 is formed of an aqueous phase in which:                soluble mineral salts can be dissolved to increase the real density of the aqueous phase to achieve a required density,        powder clays can be dispersed to create a suspension with an apparent density equal to the required density,        both of the above mentioned means, namely solubilisation of salts to increase the density of the aqueous phase and the dispersion of clays in the higher density aqueous phase, can be combined to increase the stability of the suspension of clays in the densified aqueous phase, within the limit of what is possible.        
For example, for the treatment of minerals, density methods carried out consist of performing a mechanical grinding action to the liberation mesh size and then separating the ore from the gangue in an aqueous suspension of dispersed powder clay, the clay acting as a densifying agent, in the appropriate concentration to enable separation of components to be separated into two phases, one supernatant phase and the other sinking.
However, the stability of the said suspensions at clay concentrations used to prepare densified separative suspensions is not very good because clay concentrations can vary with time depending on the variation of the composition of the gangue present and to be eliminated for which the density is not constant and varies depending on the extraction zone, and the density of the ore to be extracted is not constant either and also varies depending on the extraction area.
The density threshold chosen for the separation is relatively coarse with these methods, and usually the separation sensitivity cannot be better than a single decimal figure, in other words a densified separation medium for which the apparent density will be between 1.4 and 1.5.
For example, separation methods based on the use of dense liquid media are used, in which a sufficient quantity of mineral salts is dissolved in the aqueous phase or a sufficient quantity of powder clays is dispersed in order to determine the separation threshold density in the presence of surfactants, for synthetic organic waste materials to be reused such as fragmented polymer materials to be separated from each other or to be separated from their pollutants.
Documents have described such dense liquid separation media. A first document (DE19964175) describes a technology for the separation of a heterogeneous mix of fragmented polymer materials depending on their apparent density, in a saline aqueous solution forming the dense liquid medium with a predetermined density, the separation being done at laminar stream requiring optimisation of the stream conditions of the saline aqueous solution and the mix of polymer materials to be separated.
The method thus described only appears suitable for a single density value because it appears difficult to vary the density of the dense liquid medium within a wide range of values, this variation being related to the fact that increasing concentrations of dissolved salts are necessary to achieve relatively high densities; consequently, the operating cost of this method can then become unacceptable for the separation of waste synthetic organic materials with very low added value for reuse, particularly because the technology used is affected by obvious corrosive action due to the saline liquid medium.
Another document (U.S. Pat. No. 3,857,489) describes a method for separation of a stream of materials to be separated in a dense aqueous medium made using a powder clay suspension dispersed in water for which the stability is improved due to the addition of a water soluble hetero polysaccharide type of surfactant into the suspension to stabilise the clay. But the separative suspension is not sufficiently stable since a clay particles settlement phenomenon occurs which causes a large variation in the density threshold chosen for the selective separation.
As mentioned above, another document (EP 0918 606 B1) describes a method for the separation of fragmented polymer materials originating from the destruction of cars and/or other objects at the end of their lives by grinding that uses separation on a stream composed of a mix of filled or unfilled polymers and/or copolymers with densities ranging from 1 to 1.6, in a dense liquid medium using a suspension made of a powder clay dispersed in water so as to obtain a density separation medium with a given density threshold. But as has been mentioned, such a separative suspension has disadvantages that are difficult to overcome, for example variation of the rheology and density with time, settlement of the powder clay and the impossibility of fixing a precise density threshold, for example as far as the second decimal figure, and maintaining it.
Another document (U.S. Pat. No. 4,460,788 B1) describes a method of separating all categories of polymers originating from wastes including a grinding step, a mechanical separation step and two density separation phases in which the density varies upwards or downwards.
In the density separation phase, the liquid medium in the hydraulic separator contains an aqueous phase, a powder inorganic compound that is clay put in suspension to increase the density of water, and a wetting agent (based on a fatty alcohol modified with ethylene oxide and/or propylene oxide) that also maintains the inorganic compound in suspension. With this separation method, the size of particles in suspension is never mentioned either through a size grading cut-off or through a median diametric dimension of these particles; this lack of precision is a problem that particularly affects the stability of the suspension and more particularly the stability of the density required for this suspension, due to the obvious segregation of the largest particles that sink to the bottom of the container.
Another document (U.S. Pat. No. 6,335,376 B1) describes a method of separating polymers present in a mix of different polymers containing neither metals nor pollutants to be eliminated such as glass, foam, wood or other. The claimed method consists of heating the mix of these different polymers after dimensional selection, to cause bubbling of at least of the polymers of the said mix to be separated and then separating it from the said mix due to a change in the apparent density of the said polymer caused by heating.
This modification of the apparent density creates a differentiation between at least one of the polymers in the heated mix and the other polymers in the mix and a selective separation of alterations of the densities of each polymer in successive steps.
This method uses particular separation means that apply heat to destructure the polymer materials to be separated, instead of using an aqueous suspension of solid powder particles to create a separation density level.
Thus, the state of the art proposes different methods for the separation of materials, such as materials originating from the destruction of cars and/or durable consumer products at the end of their lives by grinding such as household appliances, electronic or other equipment which, after metals have been recovered, are synthetic organic materials, using dense liquid media with all the disadvantages mentioned above.
But the state of the art does not include any methods for the particularly precise separation of the different types of constituents with a density equal to at least 1 and usually much greater than 1, forming a mix of fragmented synthetic organic waste materials to be reused.